fix: not throw warnings when using implicit fallback

packages/core-base/src/context.ts

@@ -692,4 +692,32 @@ export function updateFallbackLocale<Message = string>(
   ctx.localeFallbacker<Message>(ctx, fallback, locale)
 }
 
+/** @internal */
+export function isAlmostSameLocale(
+  locale: Locale,
+  compareLocale: Locale
+): boolean {
+  if (locale === compareLocale) return false
+  return locale.split('-')[0] === compareLocale.split('-')[0]
+}
+
+/** @internal */
+export function isImplicitFallback(
+  targetLocale: Locale,
+  locales: Locale[]
+): boolean {
+  const index = locales.indexOf(targetLocale)
+  if (index === -1) {
+    return false
+  }
+
+  for (let i = index + 1; i < locales.length; i++) {
+    if (isAlmostSameLocale(targetLocale, locales[i])) {
+      return true
+    }
+  }
+
+  return false
+}
+
 /* eslint-enable @typescript-eslint/no-explicit-any */

packages/core-base/src/translate.ts

@@ -20,6 +20,8 @@ import { isMessageAST } from './compilation'
 import { createMessageContext } from './runtime'
 import {
   isTranslateFallbackWarn,
+  isAlmostSameLocale,
+  isImplicitFallback,
   handleMissing,
   NOT_REOSLVED,
   getAdditionalMeta,
@@ -839,6 +841,7 @@ function resolveMessageFormat<Messages, Message>(
     if (
       __DEV__ &&
       locale !== targetLocale &&
+      !isAlmostSameLocale(locale, targetLocale) &&
       isTranslateFallbackWarn(fallbackWarn, key)
     ) {
       onWarn(
@@ -905,15 +908,17 @@ function resolveMessageFormat<Messages, Message>(
       break
     }
 
-    const missingRet = handleMissing(
-      context as any, // eslint-disable-line @typescript-eslint/no-explicit-any
-      key,
-      targetLocale,
-      missingWarn,
-      type
-    )
-    if (missingRet !== key) {
-      format = missingRet as PathValue
+    if (!isImplicitFallback(targetLocale, locales)) {
+      const missingRet = handleMissing(
+        context as any, // eslint-disable-line @typescript-eslint/no-explicit-any
+        key,
+        targetLocale,
+        missingWarn,
+        type
+      )
+      if (missingRet !== key) {
+        format = missingRet as PathValue
+      }
     }
     from = to
   }

packages/vue-i18n-core/test/issues.test.ts

@@ -1359,6 +1359,67 @@ test('issue #1738', async () => {
   expect(wrapper.find('#te2')?.textContent).toEqual(`true - expected true`)
 })
 
+describe('issue #1768', () => {
+  test('Implicit fallback using locales', async () => {
+    const mockWarn = vi.spyOn(shared, 'warn')
+    // eslint-disable-next-line @typescript-eslint/no-empty-function
+    mockWarn.mockImplementation(() => {})
+
+    const i18n = createI18n({
+      locale: 'en-US',
+      fallbackLocale: 'en',
+      messages: {
+        en: {
+          hello: {
+            'vue-i18n': 'Hello, Vue I18n'
+          }
+        }
+      }
+    })
+
+    const App = defineComponent({
+      template: `<div>{{ $t('hello.vue-i18n') }}</div>`
+    })
+    const wrapper = await mount(App, i18n)
+
+    expect(wrapper.html()).toEqual('<div>Hello, Vue I18n</div>')
+    expect(mockWarn).toHaveBeenCalledTimes(0)
+  })
+
+  test('Explicit fallback with decision maps', async () => {
+    const mockWarn = vi.spyOn(shared, 'warn')
+    // eslint-disable-next-line @typescript-eslint/no-empty-function
+    mockWarn.mockImplementation(() => {})
+
+    const i18n = createI18n({
+      locale: 'zh-Hant',
+      fallbackLocale: {
+        'de-CH': ['fr', 'it'],
+        'zh-Hant': ['zh-Hans'],
+        'es-CL': ['es-AR'],
+        es: ['en-GB'],
+        pt: ['es-AR'],
+        default: ['en', 'da']
+      },
+      messages: {
+        zh: {
+          hello: {
+            'vue-i18n': '你好，Vue I18n'
+          }
+        }
+      }
+    })
+
+    const App = defineComponent({
+      template: `<div>{{ $t('hello.vue-i18n') }}</div>`
+    })
+    const wrapper = await mount(App, i18n)
+
+    expect(wrapper.html()).toEqual('<div>你好，Vue I18n</div>')
+    expect(mockWarn).toHaveBeenCalledTimes(0)
+  })
+})
+
 test('#1796', async () => {
   const i18n = createI18n({
     locale: 'en',